correctBrackets.js

let correctBrackets = (str) => {
	let allowedSymbols = {
		'[': 1, '{': 1, '<': 1, '(': 1, ']': 1, '}': 1, '>': 1, ')': 1
	}
	
	let brackets = { '[': ']', '{': '}', '<': '>', '(': ')' }

	let count = 0, ob = [], result = [];
	for(let c of str) {
		if(!allowedSymbols[c]) return null;
		if(brackets[c]) {
			ob.push(c);
			result.push(c);
			count++;
		} else {
			if(ob.length == 0) return null;
			result.push(brackets[ob.pop()]);
			count --;
		}
	}

	return count == 0 ? result.join('') : null;
}